Unsaturated polyester networks are well known for their thermal and mechanical properties especially when reinforced with fibers and highly filled. Such materials are used in boat hulls and automobile body parts. These resins are inherently brittle, and attempts to toughen the compositions without sacrificing thermal and mechanical properties were made. U.S. Pat. No. 4,419,487 discloses the use of carboxyl terminated reactive liquid polymers to toughen polyester resins. U.S. Pat. No. 4,530,962 discloses that epoxy resin terminal groups on low molecular weight elastomers makes the molecules sufficiently compatible with polyester that a homogeneous dispersion of elastomer particles can be achieved on cure. This dispersion resulted in decreased brittleness and increased fracture energy.
The use of piperazine compounds as activators in the curing of unsaturated polyester resins and their ability to form Michael-type addition products are described in J. of Applied Polymer Sci., Vol. 31, (1986) pp 1083-1091 and Vol. 42 (1991) pp 1371-1376.